Fractures of the dens (odontoid process) of the second cervical vertebra (C2) may result from severe forces, including rotation, across the cervical spine. Dens fractures may be classified according to the scheme of Anderson and D'Alonzo. Type I fractures occur at the tip of the dens. Type II fractures occur at the base of the dens without extension to the body of C2. Type III fractures occur in the body of C2.
Anderson and D'Alonzo Type II dens fractures are often fixed using a single cannulated lag screw and washer. While several authors have reported excellent results with this technique, pseudarthrosis rates approaching 20% or more have also been noted to occur. The mode of failure in patients progressing to pseudarthrosis appears to occur more frequently due to loss of fixation at the C2 body than within the dens itself.
Biomechanical studies using cadaveric models of Type II dens fractures fixed with a single cannulated lag screw have also demonstrated failure via anterior cut-out of the screw through the C2 body rather than through loss of fixation within the dens. This failure mode is consistent with the osseous architecture of the body of the axis vertebra, which has been shown to have a decreased cortical thickness and decreased density of trabecular bone relative to the dens itself.
A fully threaded variable pitch screw has recently been demonstrated to be a biomechanically favorable alternative to a partially threaded lag screw with a washer for fixation of Type II dens fractures. However, the reported mechanism of failure for both constructs was consistently via anterior cut-out of the screw through the body of C2.